Scale-up issues for in situ anaerobic tetrachloroethene bioremediation

For the full scale implementation of in situ anaerobic bioremediation of tetrachloroethene (PCE) in groundwater, the following issues must be addressed: which organic substrates at which concentration would be most effective in promoting dechlorination and are economical; how far the substrate, electron acceptor, and nutrients can be transported in the aquifer; and the placement of delivery and recovery wells for distributing these amendments. In a microcosm study, almost all of the tested inexpensive substrates supported reductive dechlorination of PCE through vinyl chloride (VC) under methanogenic conditions. A minimum of about 60 mg L-1 of organic carbon was needed to dechlorinate 23 micromolar PCE with a single feeding. In a second microcosm study dechlorination stopped at 1,2-dichloroethene (DCE) in microcosms fed higher concentrations of several substrates. At the highest concentrations the substrates inhibited DCE production. Three field tracer tests were conducted to evaluate methods to distribute the amendments across the aquifer. The natural groundwater gradient is not sufficient to distribute substrate evenly. Groundwater injection at 60 times the natural flux rate increased the distribution of substrate. A mixing strategy of cross-gradient injection further increased the distribution of the substrate. Ammonia-nitrogen, sulfate, and phosphate were retarded relative to the substrate and inorganic tracer